Optical signal systems wherein multiple signal channels are carried by optical fibers or other waveguides over extended distances are known to employ optical amplifiers to batch-amplify all of the signal channels simultaneously. Commercially employed optical amplifiers typically provide an uneven level of gain across their optical wavelength range. For example, erbium-doped fiber amplifiers (EDFAs) operating in the C-band (generally 1525 nm to 1575 nm) produce a well known two-peaked spectral gain profile. In a typical telecommunication optical system, for example, the gain spectrum or modulation depth of an EDFA, also referred to as the insertion loss variation, gain profile, etc., can range up to 15 dB and beyond. This is generally undesirable, as flat gain characteristics, i.e., level signal strength across the operating bandwidth of the system, typically are important in multiplexed optical systems for increased transmission distance, reduced signal-to-noise ratios and other non-linear effects, or to meet other operating requirements of the system.
It is known, therefore, to employ gain-flattened amplifiers, such as gain-flattened optical fiber amplifiers or other gain-flattened optical amplifiers. Gain-flattened amplifiers may be constructed using either active elements or passive elements to flatten the gain, i.e., to provide gain equalization, also referred to as optical equalization or gain compensation, or compensation of the spectral gain profile of the optical amplifier, etc. Optical fiber gratings and dielectric thin-film filters, for example, are well known and used commercially for gain-flattening.
It is known, for example to employ a dielectric thin-film gain-flattening filter with an optical amplifier, e.g., an EDFA or other optical fiber amplifier to construct a gain-flattened amplifier. Thin-film gain-flattening filters have advantageous properties, including low insertion loss, small size, economical design and manufacturing costs, acceptable environmental stability, etc. Dielectric thin-film filters are known to have maximum peak loss of about 6 dB (with acceptable quality for typical applications such as telecommunications, e.g., acceptably low PPEF, discussed further, below) under the current state of the art for their design and manufacture. See Recent Advances in Thin Film Filters, Robert B. Sargent. In published U.S. patent application 2003/0179997 A1 of Hwang et al., which is incorporated herein by reference in its entirety for all purposes, it is suggested to employ multiple thin-film gain-flattening filters together in series where the optical amplifier requires a peak loss exceeding the maximum peak loss of one filter, e.g., two 5 dB filters in series to provide a total gain compensation or gain correction of 10 dB. In the operative wavelength band of the amplifier, the thin-film gain-flattening filter is designed to have a transmission curve showing an attenuation profile, also referred to as loss curve, loss profile, spectral loss profile, peak loss, etc. corresponding to the gain profile of the amplifier. That is, as understood by those of ordinary skill in the art, the gain-flattening filter has a spectral loss profile corresponding to the gain profile of an optical amplifier in that it is designed to have a spectral response matching or tracking the inverse of the amplifier's gain profile. As a result, the spectrum of optical signals passing through both the amplifier and the gain-flattening filter in combination achieve roughly even amplification, i.e., nearly flat gain.
Typically, the optical amplifier producer or optical system designer specifies the desired attenuation curve, commonly referred to as the target loss profile, for a gain-flattening device to be used with an optical amplifier in the optical system. The gain-flattening device producer designs the filter to have a transmission curve matching the target loss profile as nearly as is reasonably possible. Design limitations may result in the theoretical transmission curve of the gain-flattening differing somewhat from the target curve. In addition, the actual transmission curve of the gain-flattening device may differ slightly from its theoretical transmission curve, due to the effect of manufacturing tolerances, e.g., in the case of thin-film filters, natural variations in the characteristics of the thin-films deposited to form the filter, minute substrate irregularities, etc. Thus, the actual transmission curve of the gain-flattening filter or other device will always or almost always differ slightly from the target curve. The difference over the wavelength range of interest is referred to as the insertion loss error function of the gain-flattening device, or simply its error function. The magnitude of the difference, measured typically in decibels or percentage, generally varies from one wavelength to another across the span of the transmission curve. The peak-to-peak error function or PPEF is the magnitude of the difference between the target curve and the actual (gain-flattened) transmission curve at the wavelength where the amplification gain was most under-corrected plus the magnitude of the difference between the target curve and the actual transmission curve at the wavelength where the amplification gain was most over-corrected. The purchaser of gain-flattening apparatus typically specifies a maximum permissible PPEF. Alternatively or in addition, a maximum difference, typically in decibels, may be specified for every point along the wavelength range of interest, e.g., a maximum difference of 0.2 dB (or other value) may be specified, such that the actual curve must be within 0.2 dB of the target curve at every wavelength within the range of interest.
The error functions of gain-flattening filters and other gain-flattening devices are somewhat dependent on the desired attenuation profile, and thin-film gain-flattening devices such as filters show larger error functions as the attenuation profile or modulation depth becomes larger or more complex. Typically, the PPEF is approximately 10% or less of the modulation depth. Thus, for example, if one or more thin-film gain-flattening filters are used to correct 6 dB of modulation depth, the PPEF can be expected to be about 0.6 dB, and correcting 12 dB of modulation depth will result in a PPEF of about 1.2 dB. In this regard it is a problem that if multiple thin-film gain-flattening filters are used in series, such as suggested by Hwang et al. cited above, for example, two thin-film gain-flattening filters each correcting 6 dB of modulation depth for a total gain compensation of 12 dB, the error function and PPEF tend to accumulate. The error function tends to accumulate because thin-film gain-flattening filters typically have systematic error functions. That is, the gain correction error as a function of wavelength will be similar or even nearly identical from one thin-film gain-flattening filter to the next, especially in the typical case of using multiple filters designed to the same target loss profile and/or manufactured in the same batch, i.e., from the same wafer. A stack of thin-films is deposited in sequence onto the surface of a large wafer that is transparent in the wavelength band of interest, resulting in nearly uniform filter properties across the wafer surface. The wafer is then diced into small pieces, e.g., 1.0 mm by 1.0 mm up to 2.0 mm by 2.0 mm or larger. The chips can then be packaged in a suitable housing, optionally together with other components for the gain-flattening filter apparatus and/or gain-flattened amplifier, such as, e.g., collimating lenses, isolators, ferrules, monitor ports, taps or mux/demux components for adding or dropping channels, supervisory channels, etc. As a result, however, thin-film gain-flattening filters from the same batch have not only the same transmission curve, i.e., the same or similar gain correction performance, but also the same or similar error function. That is the gain correction error as a function of wavelength will be nearly identical from one component to the next as a consequence of their batch manufacturing process. Consequently, cascading gain-flattening filters having such systematic errors, either by packaging multiple filters into a common housing or positioning multiple discrete gain-flattening filters in series along an optical fiber path, will cause an accumulation of error. See Gain-flattening of High-Performance Optical Amplifiers, Arkell W. Farr, Teraxion Inc. (Cap-Rouge, Canada). Similar error function concerns are raised with other gain-flattening devices.
It is an object of the present invention to provide improved gain-flattening apparatus and methods. It is another object to provide improved methods of designing and producing gain-flattening apparatus. Additional objects and advantages of the present invention will be apparent from the following disclosure of the invention and from the detailed description of certain exemplary embodiments.